Digestive System Processes and Chemical Digestion

Digestive System

• Digestive Processes:
  • Ingestion: Taking food into the digestive tract.
  • Propulsion: Moving food through the digestive tract, including swallowing and peristalsis.
  • Mechanical Breakdown: Mastication (chewing), mixing food with saliva by the tongue, churning in the stomach, and segmentation.
  • Digestion: Enzymes break down complex food into simpler molecules.
  • Absorption: Passage of digested food from the lumen of the GI tract into blood or lymph.
  • Defecation: Elimination of indigestible substances.

Chemical Digestion

• Carbohydrates:

  • Salivary amylase in saliva breaks down starch into oligosaccharides.
  • Pancreatic amylase further breaks down starch and glycogen into oligosaccharides and disaccharides.
  • Brush border enzymes convert oligosaccharides and disaccharides into monosaccharides.
  • Monosaccharides are absorbed via secondary active transport and exit via facilitated diffusion into capillaries.

• Proteins:

  • Pepsinogen, activated to pepsin in the stomach, breaks proteins into polypeptide chains.
  • Pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase) further break down polypeptides into smaller chains.
  • Brush border enzymes (peptidases) break them into amino acids.
  • Amino acids are absorbed via secondary active transport and exit via facilitated diffusion into capillaries.

• Lipids:

  • Emulsification by bile salts in the duodenum breaks large fat globules into smaller droplets, increasing surface area for lipase action.
  • Pancreatic lipase hydrolyzes triglycerides into monoglycerides and fatty acids.
  • Micelle formation: Products of fat digestion (monoglycerides and fatty acids) combine with bile salts to form tiny micelles. These ferry contents to the epithelial cells.
  • Diffusion: Fatty acids and monoglycerides diffuse into epithelial cells.
  • Chylomicron formation: Fatty acids and monoglycerides are recombined and packaged with other fatty substances and proteins into chylomicrons.

Peritoneum and Digestive Activity

• Peritoneum: Serous membrane lining the abdominal cavity, including visceral (covering organs) and parietal (lining the wall) peritoneum.
• Mesentery: Double layer of peritoneum routing blood vessels, lymphatics, and nerves, holding organs in place.
• Intrinsic Nerve Supply (Short Reflexes): Controlled by the enteric nervous system (submucosal and myenteric nerve plexuses).
• Extrinsic Nerve Supply (Long Reflexes): Linked to the central nervous system (CNS) via afferent and efferent fibers, controlling digestive activity (sympathetic inhibits, parasympathetic stimulates).

Oral Cavity (Saliva, Chewing & Swallowing)

• Saliva Function: Cleanses the mouth, dissolves food chemicals for taste, moistens food, forms a bolus, and begins starch digestion with enzymes.
• Saliva Composition: 97-99.5% water, electrolytes, digestive enzymes (salivary amylase), and proteins.
• Chewing/Mastication: The process of mechanical breakdown of food, with the help of teeth, and tongue.
• Swallowing/Deglutition: The process that takes food from buccal to esophagus, using different phases (buccal, pharyngeal & esophageal).

Gastric Gland and Secretion

• Mucosal Barrier: Protects the stomach lining from harsh digestive conditions (thick layer of bicarbonate-rich mucus, tight junctions). Damaged cells are quickly replaced.
• Parietal Cells: Secrete hydrochloric acid (HCl) for protein denaturation and pepsin activation, and intrinsic factor for vitamin B12 absorption.
• Chief Cells: Secrete pepsinogen, which is activated to pepsin by HCl and pepsin itself (positive feedback).
• Enteroendocrine Cells: Secrete gastrin for regulating stomach secretion and motility.
• Phases of Gastric Secretion: Cephalic phase (before food), gastric phase (food is in the stomach), and intestinal phase (in the small intestine).

Small Intestine Modifications

• Circular folds (plicae circulares): Permanent folds in the mucosa and submucosa that slow movement and provide more absorption surface.
• Villi: Finger-like projections of the mucosa, containing capillaries and lacteals for nutrient absorption.
• Microvilli: Microscopic projections on the surface of villi (brush border) with enzymes for further digestion.

Liver and Pancreatic Juice

• Liver Histology: Structure of liver, including portal triad (hepatic artery, portal vein, bile duct) and hepatocytes.
• Hepatocyte Functions: Processing blood-borne nutrients, storing fat-soluble vitamins, performing detoxification, and producing bile for emulsification.
• Pancreas: Exocrine function (pancreatic juice) to neutralize chyme. Contains enzymes like proteases, amylase, lipases and nucleases. The enzymes are secreted in inactive form and activated in the small intestine.

Large Intestine

• Bacterial Flora: Assists with energy recovery from indigestible foods (such as cellulose) through fermentation.
• Defecation: The propulsion of feces to the anus, stimulated by stretch receptors in the rectum.

Metabolism

• Carbohydrates: Dietary sources (starch, sugars, cellulose) used for fuel (ATP production) and stored as glycogen/fat.
• Lipids: Dietary sources (triglycerides, saturated/unsaturated fats) used for fuel, stored as adipose tissue.
• Proteins: Dietary sources (complete & incomplete proteins) used for structural components, functional molecules (enzymes, hormones, antibodies), and fuel when carbohydrates and fats are limited.
• Carbohydrate Metabolism Pathways: Glycolysis, Krebs Cycle, and Electron Transport Chain to produce ATP.

Urinary System

• Kidney Functions: Regulating water volume and solute concentration, ensuring acid-base balance, removing metabolic wastes, and endocrine functions (Renin, Erythropoietin, Vitamin D).

• Nephron Structure:

  • Renal Corpuscle: Glomerulus (capillaries) and glomerular capsule (Bowman's capsule).
  • Renal Tubule & Collecting Duct: Proximal convoluted tubule (PCT), nephron loop (descending & ascending limbs), distal convoluted tubule (DCT) and collecting ducts.

• Capillary Beds Associated with Nephrons: Glomerulus (filtration); peritubular capillaries; and vasa recta (for concentration).

• Juxtaglomerular Complex (JGC): Macula densa cells and granular (JG cells) monitor NaCl and blood pressure to regulate glomerular filtration rate (GFR).

• Renal Processes: Glomerular filtration (filtration), tubular reabsorption (reclaiming essential materials), and tubular secretion (removal of waste).

• Tubular Reabsorption:

• Tubular secretion Important way for kidneys to clear plasma of substances such as drugs or metabolites or nitrogenous wastes or excess K+.

• Urine Formation: Formation and concentration of urine dependent on levels of ADH and amount of water intake.

• Renal Clearance: Volume of plasma the kidneys clear of a substance in a given time (inulin, creatinine, etc.).